This invention relates to inkjet printing, and more particularly, to a laser-actuatable inkjet printing system and printer.
A prior art inkjet printer typically includes at least one printing cartridge or pen in which small droplets of ink are formed and ejected toward a printing medium. Such pens include printheads with orifice or nozzle plates having very small nozzles through which the ink droplets are ejected. Ejection of an ink droplet through a nozzle may be accomplished by quickly agitating a volume of ink adjacent to each nozzle. The agitation of ink produces an effect that forces a drop of ink through the nozzle.
One method of agitating the ink is by heating the ink with a transducer, such as a resistor, that is aligned adjacent to the nozzle. Printheads using such a technique typically has many drop generators, each of which includes a single nozzle aligned over an ink chamber that supports a resister. Such a drop generator is activated or fired in either a single-drop per pixel or multi-drop per pixel print mode. In the single-drop or binary mode, one ink drop is selectively fired from each nozzle toward a respective target pixel. For printing a pixel of a specific hue, the pixel might get one drop of yellow ink from a nozzle and two drops of cyan ink from another nozzle. To improve saturation and resolution, the multi-drop mode is used where two drops of yellow ink and four drops of cyan ink might be deposited on a target pixel to attain that particular hue. In both modes, only a single drop of ink is ejected onto the target pixel each time a drop generator is fired. Each subsequent drop is deposited on the target over a drop that has been previously deposited.
Another method of agitating the ink is with a laser beam. Such a method is disclosed in A. C. Tam and W. D. Gill, “Photoacoustic ejection from a nozzle (PEN) for drop-on-demand ink jet printing”, Applied Optics, Vol. 21, No. 11, pages 1891-1892. According to the method, a laser beam is focused on ink adjacent to a nozzle to generate an ultrasonic pulse. The ultrasonic pulse on arrival at the nozzle causes a single ink drop of ink to be ejected from the nozzle.
For both methods, print quality depends on the accuracy of ink agitation adjacent to an inlet of each nozzle. In the case of thermal inkjet printing where resistors are used to heat the ink, the print quality depends on the accuracy of alignment or registration of spaced-apart nozzles over ink chambers where the resistors are located. And in the case of inkjet printing using a laser beam, the print quality depends on positioning accuracy of the laser beam over spaced-apart nozzles. Any misalignment in either case would result in an insufficient amount of ink being ejected from a nozzle. Such a misalignment will be exacerbated if there is thermal expansion of the nozzle plate and will get worse with increasing length of the nozzle plate.
Thermal inkjet printheads, such as the printhead disclosed in U.S. Pat. No. 6,099,108, Weber et al., “Method and Apparatus for Improved Ink-drop Distribution in Ink-jet Printing”, with two or more nozzles per drop generator nevertheless suffer from the same disadvantage.